Positive displacement gear pumps are commonly used to pump moderate to high viscosity liquids. A typical positive displacement gear pump includes a rotor gear mounted on a shaft; the rotor gear contains a plurality of circumferentially disposed, spaced-apart, radially inwardly directed gear teeth that also extend axially toward an open end of the pump casing. A head covers the open end of the pump casing, and the head supports an idler pin to which an idler gear is mounted eccentrically with respect to the rotor gear. The idler gear also contains a plurality of gear teeth circumferentially disposed between successive idler gear roots. In contrast to the rotor gear teeth, which extend radially inwardly, the idler gear teeth extend radially outwardly.
A crescent-shaped seal is disposed radially between unmeshed teeth of the idler gear and rotor gears, the seal being positioned within a crescent-shaped gap, generally directly opposite a point of fully engaged meshing rotor and idler gear teeth. The crescent seal is necessary to assure sufficient pressure differentials between an inlet (suction) port and an outlet (discharge) port of the pump. The idler gear teeth engage an inboard, radially inwardly curved, portion of the seal, while the rotor gear teeth engage an outboard, radially outwardly curved, portion of the seal. In addition, the intermeshing idler and rotor teeth also act as a seal between the inlet and outlet ports. Thus, sealing effects of the intermeshing teeth, as well as of the crescent seal, cooperate to retain desirable pressure differentials between the inlet and outlet ports.
Although considerable progress has been made in sealing technologies related to positive displacement gear pumps, additional improvements are needed. For example, in pumping of slurries that include growing particles, such as crystals suspended in liquid slurries, idler and rotor gear teeth often undesirably crush the suspended particles.
Thus, there is a particular need to avoid crushing of suspended particles, as for example sugar crystals within a sugar slurry during their movements through a positive displacement gear pump.